NaOsO 3 hosts a rare manifestation of a metal-insulator transition driven by magnetic correlations, placing the magnetic exchange interactions in a central role. We use resonant inelastic x-ray scattering to directly probe these magnetic exchange interactions. A dispersive and strongly gapped (58 meV) excitation is observed indicating appreciable spin-orbit coupling in this 5d 3 system. The excitation is well described within a minimal model Hamiltonian with strong anisotropy and Heisenberg exchange (J 1 = J 2 = 13.9 meV). As a result, the observed behavior places NaOsO 3 on the boundary between localized and itinerant magnetism.